Storage tube comprising electro-luminescent phosphor and cadmium sulfide field sustained conducting target

ABSTRACT

This invention provides a high resolution direct view storage tube which, in addition to having superior resolution capabilities, also requires less complex circuitry than standard type direct view storage tubes. More particularly, a meshless storage tube is disclosed which makes use of a meshless multilayered thin film structure as a combined image storage and display medium. The principle components of this structure include a thin film electronic storage medium, an opaque antifeedback layer, and an electroluminescent image display layer. In operation, a bias voltage is maintained across the entire structure, and a high resolution image is initially impressed upon the storage medium by means of a scanning electron beam. The scanning beam creates local conductivity modulations within the storage film which correspond to the input image, and these modulations, in turn, alter local field configurations across the electroluminescent layer, thus creating a visible output image. After initial scanning, the conductivity modulations in the storage layer are maintained for an extended period of time (several tens of seconds) provided that an applied electric field continues to be maintained. This phenomenon is hereinafter referred to as &#39;&#39;&#39;&#39;field sustained conductivity.&#39;&#39;&#39;&#39; Removal or reversal of the applied electrical field restores the storage element to its initial insulating condition, and the screen is thereby erased.

United States Patent Bleha, Jr. et al.

[ 1 Aug. 1, 1972 [54] STORAGE TUBE COMPRISING ELECTRO-LUMINESCENTPHOSPHOR AND CADMIUM SULFIDE FIELD SUSTAINED CONDUCTING TARGET [72]Inventors: William P. Bleha,Jr., Santa Monica; Ronald F. Scholl, Malibu,both of Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: June 18, 1971 21 App]. No.: 154,383

52 us. c1. ..313/68 D,3l3/92R [51] Int. Cl ..H0lj 31/58, l'IOlj 31/10,l-lOlj 29/45 [58] Field of Search ..313/68 D [56] References CitedPrimary ExaminerRobert Segal Attorney-W. l-l. MacAllister, Jr. et al.

[57] ABSTRACT This invention provides a high resolution direct view Moreparticularly, a meshless storage tube is disclosed which makes use of ameshless multi-layered thin film structure as a combined image storageand display medium. The principle components of this structure include athin film electronic storage medium, an opaque antifeedback layer, andan electroluminescent image display layer. In operation, a bias voltageis maintained across the entire structure, and a high resolution imageis initially impressed upon the storage medium by means of a scanningelectron beam. The scanning beam creates local conductivity modulationswithin the storage film which correspond to the input image, and thesemodulations, in turn, alter local field configurations across theelectroluminescent layer, thus creating a visible output image. Afterinitial scanning, the conductivity modulations in the storage layer aremaintained for an extended period of time (several tens of seconds)provided that an applied electric field continues to be maintained. Thisphenomenon is hereinafter referred to as field sustained conductivity.Removal or reversal of the applied electrical field restores the storageelement to its initial insulating condition, and the screen is therebyerased.

3 Clains, 6 Drawing Figures PATENTED 1 1 I972 3 6 81, 6 3 8 sum 1 0F 5William P. Bleho,Jr., Ronald F. Scholl,

INVENTORS.

ATTORNEY.

PATENTEDAuc 1 i972 Device Current mA SHEET '4 OF 5 IOO ililii] I lllliiiliiili] iiiilii] Induced current Sustained current 7 Sample Erasecurrent I t l l- 1 Device Area 03 cm Gold -Si|icon Monoxide CompositeContact IO 7 2O 3O 40 50 Device Voltage Fig. 5.

PATENTEDmc I I972 3.681.638

sum 5 or 5 Sample thickness I25 Test oreo=.3cm Applied voltage 40Vlnduced current Erase current Sustained current 0 IO 20 3O 4O 5O 6O 7O8O 90 I00 I20 Time,$ec.

Fig.6.

STORAGE TUBE COMPRISING ELECTRO- LUMINESCENT PI-IOSPHOR AND CADMIUMSULFIDE FIELD SUSTAINED CONDUCTING TARGET BACKGROUND OF THE INVENTIONThe present invention represents an improvement in the apparatus andmethod described in US. Pat. No. 3,344,300 entitled, Field SustainedConductivity Devices With CdS Barrier Layer. In this patent, a method ofmaking a field sustained conductivity device is taught, comprising thesteps of disposing a layer of cadmium sulfide in contact with analuminum electrode member, and forming a barrier region in said layer ofcadmium sulfide by heating thealuminum electrode member and the layer ofcadmium sulfide at a temperature of from 200 to 400 C. at least 2 hoursin a sulfur-containing atmosphere. A principal disadvantage of devicesmade by this method is that they can sustain a potential difference ofonly a few volts usually less than volts, and operate at relatively lowfield-sustained current levels (tens to hundreds of micro-amps).

SUMMARY OF THE INVENTION advantages in the ease and safety offabrication, and

the economic advantage of greatly improved reproducibility of themultiple device components. Performance superiority is demonstrated by abetter than order-of-magnitude increase in display highlight brightness,increased storage times, and a marked improvement in the stability ofall device characteristics. These improvements resulted primarily fromthe modification of fabrication processes and the use of a new type ofrectifying negative electrode. The secondary results of these changesare that device fabrication is presently less complex, presents less ofa safety hazard (H 8 and other sulfur-bearing process gases are nolonger required), and the improved reproducibility of each fabricationstep has led to significantly higher yields.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic drawing of astorage tube including a cadmium sulfide thin film field sustainedconductivity storage target;

FIG. 2 illustrates apparatus used to carry out thedeposition of thecadmium sulfide film step in fabricating the device of FIG. 1;

FIG. 3 illustrates apparatus used to carry out the post depositionthermal processing step in fabricating the device of FIG. 1; a

FIG. 4 illustrates apparatus used to carry out the coevaporation of theopaque insulating layer and the composite electrode in the device ofFIG. 1; and

FIGS. 5 and 6 illustrate voltage and time versus current characteristicsof a representative device according to FIG. 1.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to thedrawings and to FIG. 1 in particular, a meshless storage tube using animproved field sustained conducting target is comprising an evacuatedenvelope 10 having a large bulbous portion 12 and a small diameter neckportion 14. An electron gun 16 is disposed in one end of the neckportion 14 and includes a cathode 18, an intensity modulating grid 20,and a beam-forming section 22. Any conventional electron gun may beutilized in the tube of the present invention and further detaileddescription-thereof is not deemed necessary herein. Disposed between theelectron gun l6 and the bulbous portion 12 and around the neck portion14 of the tube is an electromagnetic deflection yoke 24. It will beunderstood that the electron beam produced by the electron gun 16 isdirected down the neck portion 14 of the tube and through theelectromagnetic fields established by the deflection yoke 24 whereby theelectron beam may be deflected vertically and horizontally with respectto the axis of the tube 10. While an electromagnetic deflection systemhas been shown, it is also possible to utilize an electrostaticdeflection system in order to make the electron beam follow apredetermined path.

The end of the large bulb portion 12 which is opposite the neck section14 is provided with an optically transparent faceplate portion 26.Disposed on the inner surface of the faceplate 26 is a target structure30 comprising an optically transparent, electrically conductiveelectrode member 31 in the form of a thin film or layer of metal such asgold, for example, which is designated the bottom electrode. Suchtransparent conductive electrodes are well known and may be formed ofother materials than metals. Thus it is possible to utilize a thin filmof tin oxide, commercially known as NESA, for this purpose. Disposed onthe transparent electrode layer 31 is a layer 32 of phosphor material ofthe type capable of having its light output modulated or of luminescingwhen subjected to a dc electric field. A typically suitable phosphormaterial of this electroluminescent type may be ZnSzCu, Cl, or Mn. Amore detailed description of electroluminescent phosphors suitable foruse in the tube of the present invention and their preparation is foundin an article by P. Goldberg and J. W. Nickerson in the Journal ofApplied Physics (1963), Vol. 34, at page 1,601. A thin opaque insulatinglayer 33 is disposed over the electroluminescent layer 32.

A thin film or layer 34 of field sustained conductive material isdisposed over the opaque insulating layer 33. A two-layer electrode filmor combined layer 36, which may be designated the top electrode, issuperposed on the field sustained conductivity layer 34 and connectionsto external voltage sources 40 and 42 are made through the wall of thetube 10 so as to permit the establishment of a predetermined electricfield across the field sustained conductivity layer 34 as well as acrossthe electroluminescent phosphor layer 32. The top electrode 36 usuallyincludes a composite film 43 of two materials which have diverseconducting properties immediately adjacent the thin film 34 of cadmiumsulfide and a metal or other conductive film overlayer 44. Specialthree-layer or single layer contacts are sometimes used. Voltage sources40, 42 apply a potential of either polarity across electrodes 31 36.

- these conductivity changes, to integrate successive excitations and toreturn to the low conductivity state as a result of amomentary reversalor removal of an electric field applied across the semiconductor film 34by the top and bottom electrodes 31, 36 and voltage sources 40, 42. I e

The fabrication of the meshless image storage and display screenaccording to the invention may be classified into five basic processingsteps: (1) deposition of the bottom electrode 31, electroluminescentlayer 32 and opaque layer 33 onto the faceplate 26; (2) thermalprocessing of the electroluminescent layer 32, (3)

evaporation of the cadmium sulfide thin film 34 onto the opaque layer33; (4) thermal processing of the cadmium sulfide thin film 34, and (5)deposition of the top electrode 36 on the cadmium sulfide thin film 34emerging from step 3. Following this outline, the fabrication of ameshless storage tube having an improved field sustained conductivitystorage target will now be described. It will be understood that manyvariations in the process are available and that dimensions and shapeare exemplary only.

PREPARATION OF FACEPLATE 26 AND BOTTOM ELECTRODE 31 .The first step indevice fabrication is to prepare the support faceplate 26 and depositthereon the so-called bottom electrode 31. In general, a variety oftransparent conducting materials can be deposited to serve as the bottomelectrode. It is required, however, that the inner surface of faceplate26 bewell cleaned by an appropriate technique prior to the electrodedeposition. A SnO,Sb electrode on the glass faceplate 26 is a preferredmaterial since it is highly transparent. When a non-commercial vacuumdeposited bottom-electrode 31 is freshly prepared in the laboratory, itssurface requires no cleaning or other treatment in preparation for thecadmium sulfide deposition. The electroluminescent layer 32 and thinopaque insulating layer 33 are deposited over the bottom electrode 31 bystandard evaporation or co-evaporation techniques. Theelectroluminescent layer 32 is then heat-treated to achieve filmactivation.

DEPOSITION OF CdS FILM 34 The third step in the device fabrication isthe vacuum deposition of the CdS film 34. The deposition may be done inthe bell jar of a conventional high vacuum system in the pressure rangeof l x to 1 X 10" Torr. The pressure, however, does not appear to becritical. A cross-section drawing of the instrumentation is shown inFIG. 2. The vacuum is enclosed by a baseplate 50 and a glass bell jar52.'The electrode faceplate 26 is held by a stainless steel substrateholder 53 and heated'b'y quartz lamps 54. A removable shutter 55 shieldsthe faceplate 26 until deposition on it is to be commenced. Thethickness of the CdS film is directly and continuously monitored on thefaceplate by the use of optical interference. This is accomplished withthe use of a laser 56 and detector 57 positioned outside the bell jar52.

The electronic grade CdS powder, in the form of a pressed cylindricalpellet 58, is evaporated from a formed tantalum boat 60 which isresistively heated by current passing through buss bars 62 and currentfeedthroughs 64. The boat 60 is designed such that as the CdSevaporates, the pressed pellet 58 settles down into the boat. This givesan efficient thermal evapora tion over the long period of time requiredfor the deposition of the CdS films. The evaporation rate is controlledby controlling the current to the tantalum boat. The .current is set sothat 2.5;; of CdS as monitored by optical interference is depositedonthe electroded faceplate 26 in 1 hour. Typical thicknesses of CdSfilms are 5 12.5 p. so that deposition times of 2-5 hours are required.Successful results have been obtained with thicknesses from 2-15 p. andevaporation rates from 0.5 to 10.0 p/hr. To avoid the heating of thevarious elements in the deposition chamber by radiation from thetantalum boat 60,'a water cooled plate 65 is positioned beneath the boat60 and extending to the diameter of a cylindrical stainless steeldeposition chamber 66 disposed thereabout. The water-cooling is i usedto maintain the temperature of the wall of chamber 66 as measured by athermocouple 67 below 60 C. This low temperature, as compared with thefaceplate 26 temperature of 130. C., as measured by a thermocouple 68,is necessary to obtain the desired characteristics in the films. Itshould be noted, however, that the important fact is that the chamber 66and baseplate 50 are maintained at a lower temperature than thefaceplate 26 and the methods of achieving this can be determined by oneskilled in the art. Also, the temperatures given can be changed to varythe conductivity and current-voltage characteristics of the CdS films. Arange of substrate temperatures from 100 to 200 C. and chamber walltemperatures from 40 to 90 C. have been used to make CdS films of thegiven characteristics.

PosT DEPOSITION THERMAL PROCESSING The fourth step in device fabricationis the postdeposition thermal processing of the device as it emergesfrom step 3. The preferred process under the present invention can beseen by reference to FIG. 3. A controllable furnace 70 is providedwith aquartz tube 72 of suitable diameter. A gas inlet tube 74 introduces gaswhich is preheated while passing through the core of the furnace. Thegas exits through short exit tube 75. The temperature (for monitoringand control) near the center of the tube and also near the center of thehot zone is provided by a thermocouple 76 sheathed in a quartz tube 77.The electroded faceplate 26 with film 34 is placed in the tube near thecenter of the hot zone. A controllable flow of gas from a gas cylinder78 is provided by pressure regulator 79 and fiowmeter 80.

It should be recognized that other configurations obvious to thoseskilled in the art, can be used. In operation the following procedure isfollowed. First the faceplate 26 is inserted in the tube 72 and the tubeis flushed out with the gas from cylinder 78. Typically Argon is used,but other non-sulfur atmospheres have been used successfully, includingnitrogen and air.

Then the flow of Argon is typically reduced to CFH (at standardtemperature and pressure) and the furnace turned on. Flow rates from 0.lCFI-I to CFH have been used with success. The oven is brought to thedesired temperature, typically 500 C., and kept at that temperature forthe desired time, typically one minute. Temperatures from 385 to 525 C.and times from 1 to 60 minutes have been successfully used. Theparticular time and temperature used depends on the thickness of the CdSfilm 34, the substrate material, and the type of gas used. Also thedevice characteristics, for a given thickness of CdS film, substratematerial, and gas, can be altered by changing the temperature and time.After the desired time has elapsed, the quartz tube 72 is physicallyremoved from the furnace 70 and allowed to cool in 20 minutes to 70 C.,at which point the faceplate 26 is removed.

While this rapid cooling produces superior results, devices exhibitingthe desired characteristics can also be obtained by leaving the quartztube 72 in the furnace 70 and turning off .the power to the furnace.Under these circumstances, the faceplate 26 cools down about a factor of10 more slowly.

DEPOSITION OF TOP ELECTRODE 36 I The final step in device fabrication isto apply the top electrode 36 to the device as it emerges from step 3.In the present device the top electrode'36 is usually made up of twolayers rather than a single layer. The first layer 43 applied of thedouble layered electrode 36, FIG. 1, is a composite film of twomaterials which have diverse conducting properties (metal/dielectric,metal/semiconductor, semiconductor/dielectric, etc.) and the secondlayer 44, FIG. 1, is a simple metal film overlayer. Negative contact ismade to the device via the metal overlayer 44.

The preferred type of composite film 43 has been a mixed coevaporatedlayer of gold and silicon monoxide. This film is prepared in a vacuumchamber 82 such as shown in FIG. 4. In practice the Au is evaporatedusing an electron beam evaporator 83 and the rate of Au evaporation ismeasured and controlled by a rate monitor 84. Similarly, the SiO isevaporated from a Drumheller source 85 and the rate of SiO evaporationis measured and controlled by rate monitor 86. Although the evaporationstake place simultaneously, an optical shield 87 prevents each ratemonitor from sensing any of the evaporant from the other source. Thisshield 87 does, however, allow the evaporant streams to mix in region 88of the chamber 85. It is in this region 88 that composite filmdeposition occurs. The faceplate 26 as it emerges from step 3 is placedin a rotating substrate holder 90 shielded by a shutter 92 and thechamber 82 is pumper to approximately 10' Torr. The rates of theindividual evaporants are then set to a predetermined level (whichcontrols their relative composition in the deposited film), the shutter92 is opened, and the film is deposited for a fixed time at the presentrates to yield the desired thickness. Typical films are on the order of2,500 A. thick and contain a few percent Au, but other compositions andthicknesses may also be used. Over this composite film 43 a continuousconducting electrode 44 is then deposited to complete the device.Negative contact is made to the device via the metal overlayer 44 of thetop electrode 36. The preferred type of composite film has been a mixedcoevaporated layer of gold and silicon monoxide. However, other metalshave been successfully substituted for gold, such as aluminum, silver,platinum and tin and other dielectrics have been substituted for siliconmonoxide, such as, for example, magnesium oxide. Semiconductor materialssuch as germanium have also been substituted for the metallic element inthe composite film with good results. In addition to the coevaporationtechnique for obtaining the composite film, three other techniques havealso been used with good success. One technique is to precipitate amonolayer of metal particles on the surface 'of the cadmium sulfide thinfilm 34 and then apply an overlayer of a dielectric such as siliconmonoxide or an overlayer of a semiconductor such as cadmium telluride.Another technique is to first evaporate a very thin discontinuous metalfilm onto the cadmium sulfide surface followed by an overlayer ofdielectric. Each of these techniques require a final overlayer ofconducting metal.

The above contacting techniques have the common feature that the filmsurface immediately adjacent to the cadmium sulfide film in all casesconsists of islands or patches of a material of one conductivity type(for example, metal) surrounded by regions of a material of a diverseconductivity type (for example, dielectric). It is this common uniquefeature which, when combined with the cadmium sulfide film as preparedabove, gives rise to the enhanced sustained conductivity effects foundin the field sustained conductivity device of the present invention.

DEVICE CHARACTERISTICS The storage characteristics of the device can beseen with reference to FIGS. 5 and 6. In FIG. 5 is shown acurrent-voltage characteristic of the device with the polarity ofapplied dc voltage as shown in FIG. 1. This is with the bottom electrode31 biased positively with respect to the top electrode 36. In FIG. 5 theinduced current is the current flowing through the device for a fixedvoltage when an electron beam is incident on it. The sustained current102 is the current flowing through the device for a fixed voltage, 5sec. after the electron beam is removed. The erase current 103 is thecurrent that flows through the device 5 sec. after the fixed voltage hasbeen momentarily reduced to zero or made negative. It has been observedthat the voltage can be momentarily reduced to zero for as little as 10milliseconds and still retain this erase current 103 characteristic. Itshould be noted that these characteristics show a much higher levelvoltage operation and also higher sustained current 102 than incontemporary devices. In FIG. 6 is shown the behavior of the currentthrough the device as a function of time. A fixed voltage of 40V isacross the device. At time t 0 sec. the induced current 105 caused by anincident electron beam is indicated. After I 0 sec. the electron beam isremoved and the sustained current level 106 (with 40V still across thedevice) is shown. At time t 28 sec. the voltage across the device ischanged to zero and no current flows. At time 6 35 sec. the voltage ischanged back to 40V and the erase current 107 is shown until 6 sec. Itshould be noted that the erase current 107 remains a smaller fraction ofthe sustained 7 current 106 for a longer time than in contemporarydevices.

It should also be noted that the characteristics in FIGS. and 6 aretypical of a particular processing schedule of the device. ,By varyingparameters such as thickness of the CdS film 6, the time and temperatureof post deposition thermal processing, and the type of (0.3 cm area; 5sec. after removal of electron beam Erase current through device:0.00001 1 mA (0.3

cm area; 5 see. after momentary removal of voltage across device) r Inoperation, when the electron beam from the electron gun 16 modulated inaccordance with information representative signals, scans the targetstructure 30, the

field sustained conductivity layer 34 becomes conductive in apoint-to-point fashion to a degree dependent upon the modulation oftheelectron beam. The electric field is thereby increased across theelectroluminescent layer 32 in a similar point-topoint fashion causingthis phosphor layer to luminesce and produce a visual presentationcorresponding to the information to be displayed. Because theconductivity of the layer 34 is sustained even after bombardment thereofby the scanning electron beam has ceased, the electroluminescentphosphor layer remains excited and continues to luminesce at averagebrightnesses of l 4 ft. L. Stored displays may be erased by dependingupon the decay of conductivity in the field sustained conductivity layer34, the time of which may range from seconds to minutes. It is alsopossible to restore the barded state-at any time by momentarilyreversing the electric field applied thereacross. The opaque insulatving layer 33 operates to avoid any optical feedback between the fieldsustained conducitivity layer 34 and i the'electroluminescent layer 32.The opaque layer 33 may comprise a thin film of germanium or a cermet ofgermanium and silicon monoxide, for example, and effectively preventsphotons produced by the electroluminescent layer 32 from feeding back orotherwise reaching the field sustained conductivity layer 34 andadversely affecting its conductivity. What is claimed is:

1. A direct-viewing electronic storage display device stora ela er 'ncldi 1 l. a layer 0 ca mi u m sulfide disposed on said opaque layer, saidlayer of cadmium sulfide being heated at a temperature of from 385 to525C. for a period of from 1 minute to 1 hour in a non-sulfur-containingatmosphere followed by immediate cooling in said atmosphere; anelectrode member in the form of a composite layer of two materials whichhave diverse conducting properties disposed on said heat treated layerof cadmium sulfide, one of said materials being selected from a groupconsisting of gold, aluminum, silver, platinum, tin and germanium,

and the other of said materials being selected from the group consistingof silicon monoxide and magnesium oxide; and

3. a conductive film disposed over said composite I layer; e. and anelectron gun disposed in said container, for

forming an electron beam of elemental cross-secv tional area and adaptedto scan said composite storage layer. 2. The invention according toclaim 1 wherein said non-sulfur-containing atmosphere in which saidlayer of cadmium sulfide is heated is a gas selected from a group ofgases consisting essentially of argon, nitrogen and air.- g

3. The invention according to claim 1 wherein said layer of cadmiumsulfide is from 2 to 15 microns thick.

* l I I

2. an electrode member in the form of a composite layer of two materialswhich have diverse conducting properties disposed on said heat treatedlayer of cadmium sulfide, one of said materials being selected from agroup consisting of gold, aluminum, silver, platinum, tin and germanium,and the other of said materials being selected from the group consistingof silicon monoxide and magnesium oxide; and
 2. The invention accordingto claim 1 wherein said non-sulfur-containing atmosphere in which saidlayer of cadmium sulfide is heated is a gas selected from a group ofgases consisting essentially of argon, nitrogen and air.
 3. Theinvention according to claim 1 wherein said layer of cadmium sulfide isfrom 2 to 15 microns thick.
 3. a conductive film disposed over saidcomposite layer; e. and an electron gun disposed in said container forforming an electron beam of elemental cross-sectional area and adaptedto scan said composite storage layer.